The long term goal is to understand the molecular mechanism of hepatocarcinogenesis so that novel strategies for the effective prevention and treatment of liver cancer can be developed. Epigenetic abnormity is a major hallmark in hepatocellular carcinogenesis. The histone methyltransferase enhancer of zeste homolog 2 (EZH2) plays a key role in embryonic development via catalyzing trimethylation of histone H3 at lysine-27 and facilitating DNA methylation. EZH2 is overexpressed in human liver cancer, and knockdown of EZH2 causes apoptosis of liver cancer cells and tumor regression in nude mice. However, little is known about the in vivo importance of EZH2 in liver pathophysiology and carcinogenesis, largely due to the embryonic lethality of EZH2-null mice. Additionally, activation of the oncogene c-Myc causes liver cancer in mice, and c-Myc overexpression is associated with malignant transformation of hepatocytes in humans; however, the underlying mechanism of c-Myc-induced hepatocarcinogenesis remains unclear. The objective of this proposal is to elucidate the pathophysiological importance of EZH2 in liver utilizing novel models of mice with liver-specific knockout (LKO) of EZH2 and/or overexpression of c-Myc. We found that EZH2-LKO mice appeared normal, suggesting that loss of EZH2 in liver is well-tolerated. EZH2 is essential for the regeneration of pancreatic beta cells. We found that EZH2 was markedly induced during liver regeneration, suggesting its importance for the regeneration of liver. The central hypothesis is that EZH2 is important for liver regeneration and is required for c-Myc-induced hepatocarcinogenesis through inhibiting c-Myc-induced cell senescence/apoptosis and promoting c-Myc-induced cell proliferation. Knockout of EZH2 will cause a significant delay of liver regeneration and blockage of c-Myc-induced hepatocarcinogenesis. This central hypothesis will be tested in two specific aims. Aim 1 will determine the importance of EZH2 in liver regeneration using models of 2/3 partial hepatectomy in EZH2-LKO mice. The working hypothesis is that EZH2-LKO mice will have considerable delay in liver regeneration but will eventually regenerate the liver due to compensatory mechanism(s). Aim 2 will determine the importance of EZH2 in c-Myc-induced liver carcinogenesis using mice with liver-specific knockout of EZH2 but overexpression of c-Myc. The working hypothesis is that EZH2 deficiency will prevent c-Myc-induced liver cancer via inhibiting cell proliferation but enhancing cell apoptosis/senescence. Results from this study will provide crucial novel knowledge about the importance of EZH2 in liver regeneration and liver carcinogenesis, which may ultimately help to identify EZH2 as a novel target for the prevention and treatment of liver cancer, a deadly disease with increasing incidence in the USA. Moreover, understanding the role of EZH2 in liver regeneration will help to determine how EZH2 inhibitors may influence liver injury-repair and the safety of chemotherapy when EZH2 inhibitors are combined with other anticancer drugs to treat diverse malignancies.